Radar sensors are, for example, employed in ACC (adaptive cruise control) systems for motor vehicles and are used to determine the positions and relative speeds of radar objects, so that, as part of an adaptive cruise control, the speed of one's own vehicle may be subsequently adjusted to the speed of a preceding vehicle and the distance to the preceding vehicle may be controlled to an appropriate value.
Such a radar sensor has, for example, multiple channels which are each assigned at least one antenna element having a particular directional characteristic and an evaluation device. The high-frequency signal received in the different channels are converted by being mixed with a reference frequency, by maintaining the phase and amplitude relations, into low-frequency signals, which are evaluated in an electronic evaluation unit. For example, the low-frequency signals may be digitized using analog/digital converters and then further processed digitally. By evaluating the phase relations and/or by evaluating the amplitude relations between the different channels, it is possible to determine the azimuth angle of a located radar object.
Each radar object is plotted in the frequency spectrum of a channel in the form of a peak whose position is a function of the Doppler shift and thus of the relative speed of the radar object. If the transmission frequency of the radar system is modulated, as it is the case in an FMCW (Frequency Modulated Continuous Wave) radar, for example, then the position of the peaks is also a function of the propagation time of the radar signals. If the transmitted signal has different, appropriately selected frequency ramps within one measuring cycle, it is possible to compute the relative speed of the object and the distance of the object from the frequencies of the obtained peaks.
German Published Patent Application No. 10 2004 036 580 describes a method and a device for detecting an object in a vehicle which is equipped with an object sensor system which emits electromagnetic waves and receives the waves reflected off the objects within the detection range. A moving object in the form of a preceding vehicle is measured with the aid of a direct object reflection in which the microwave radiation emitted by the object sensor system is reflected off the object and is immediately received again by the object sensor system. In a situation in which an object, e.g., a guardrail, extending along the roadway is present on which another reflection of the measuring radiation takes place, the measuring radiation emitted by the object sensor system may be reflected off the preceding vehicle and then again off the guardrail, and received by the object sensor system as an indirect object reflection. This indirect object reflection is recognized by the object sensor system as a phantom target, since the roadway edge is estimated or measured and it is recognized that the phantom target is beyond the roadway edge. A seemingly real object location which is computable with the aid of the location of the phantom target and with the aid of a computed reflection point on the guardrail is used to check the directly measured object reflections for plausibility.